1. Field of the Invention
This invention relates to wireless communication systems and, more particularly, to a wireless communication system using modulated backscatter technology.
2. Description of the Related Art
Radio Frequency Identification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags. In RFID systems, the Interrogator communicates to the Tags using modulated radio signals, and the Tags respond with modulated radio signals. After transmitting a message to the Tag (called the Downlink), the Interrogator then transmits a Continuous-Wave (CW) radio signal to the Tag. The Tag then modulates the CW signal using modulated backscattering where the antenna is electrically switched, by the modulating signal, from being an absorber of RF radiation to being a reflector of RF radiation. This modulated backscatter allows communications from the Tag back to the Interrogator (called the Uplink). Conventional Modulated Backscatter (MBS) systems are designed a) to identify an object passing into range of the Interrogator, and b) to store data onto the Tag and then retrieve that data from the Tag at a later time in order to manage inventory or perform some other useful application.
Sensors are used to monitor the current state of a device. An example of a sensor application is to monitor the temperature, pressure, or other characteristic of a mechanical or biological device. Sensor technology has advanced to the point where inexpensive sensors, such as temperature, pressure, etc. can be attached to microprocessors. However, these sensors must communicate their results back to a central control unit.
In another sensor application, we desire to know the relative velocity of a sensor or Tag with respect to a base unit (Interrogator). For example, in an Electronic Toll Collection system, it may be important to not only identify the Tag and store or retrieve data from the Tag, but also determine the velocity of the Tag, perhaps to determine if the vehicle is speeding. In a security access application it would be helpful to identify an object having a Tag, determine the velocity of the Tag, and also to determine if movement is present in the reading field, regardless of whether a Tag is present.
Beyond security, other applications require the ability to monitor sensor outputs. For example, a pump may have a certain vibration "signature" during normal operation, and a different vibration signature during abnormal operation. It is important to ascertain when the pump's vibration signature changes from normal to abnormal.
In some of the described embodiments of this invention, we disclose methods for using MBS RFID systems to perform functions such as, determining the relative velocity of the Tag with respect to the Interrogator, determining if movement is present in the reading field even if not Tag is present, and determining the vibration signature of a device, such as but not limited to a pump, to which the Tag is attached. In this manner, an inexpensive RFID network, consisting of one or more Interrogators, can be constructed which: performs RFID functions, sensor functions, motion detection, and analysis of sensor data functions.